This invention relates to the stabilization of fibers of polypropylene or predominantly propylene-containing copolymers (together referred to hereafter as "PP fibers" for brevity) colored with a specific pigment namely Red 144 (common name). It is known that several stabilizers, particularly hindered amine light stabilizers ("HALS"), provide excellent stabilization of the PP fibers, but not to the red coloration of Red 144-pigmented PP fibers. These pigmented PP fibers lose their pigmentation, due to chemical degradation of the pigment, long before the fibers themselves are degraded past the point where they serve their intended use. The problem is exacerbated because increasing the concentration of Red 144 pigment in the fibers accelerates their degradation.
Red 144 (referred to as such for brevity and convenience) is an azo condensation pigment, more correctly identified as [2-napahthalenecarboxamide, N,N'-(2-chloro-p-phenylene)bis[4-[2,5-dichlorophenyl)azo]-3-hydroxy (reg. no. 5280-78-4), available as the commercial product Cromophtal Red BRN, from Ciba-Geigy, and Red BR PR144/45415 from Ampacet.
Commercially available PP fibers today have good resilience and heat stability, and have successfully been stabilized against ultraviolet (uv) light degradation with a wide spectrum of HALS. But such stabilized PP fibers have poor dyeability because PP is essentially unreactive with most dyes. This poor dyeability of PP dictates that PP fibers must be pigmented for long-term stability of PP fibers colored with many popular colors. With particular respect to red PP fibers which are in high demand, an effective red pigment now in use is Red 144. The problem is that the use of Red 144, both hastens the degradation of the PP fibers when exposed to sunlight, and degrades their physical properties over time, so that combined, the fibers are subjected to a two-pronged attack on their longevity in normal use, thus vitiating their marketability.
Fabrics made from Red 144 pigmented PP fibers are especially popular in automobiles, boats, outdoor clothing, and other such uses where the fibers degrade at such an unacceptably high rate upon exposure to sunlight, that they are soon transformed into nonuniformly colored articles sporting a wide spectrum of shades of pink and orange. The obvious way to cope with this color degradation problem is to use far more pigment than is required to provide the desired color, so that upon suffering the expected color degradation, the coloration of the remaining non-degraded pigment will maintain acceptable, if not the original, color. Except that `loading up` the HALS-stabilized fibers with more Red 144 pigment to maintain tinctorial strength, simply accelerates degradation of the PP fibers because Red 144 has a high proclivity towards reaction with commonly used HALS, and other additives such as antioxidants and antiozonants, used to provide melt-stability to the PP.
Typically, several additives are combined in PP before it is melt extruded into fiber, each additive specifically designed to provide a different zone of stabilization, the main zones being (a) melt extrusion stability, (b) long term thermal stability during conditions expected to be encountered during use, (c) uv light stability in bright, direct sunlight, and by no means of least importance, (d) stable tinctorial strength to maintain the desired color. Combining several additives known to be effective for each specific purpose, in PP fibers particularly, is likely not to produce the desired results because of objectionable side effects due to interaction between the additives.
For example, thiodipropionate compounds such as dilauryl (DLTDP) and distearyl (DSTDP) help control melt-stability despite an odor problem, and certain phosphites control melt flow while depressing the tendency of PP fibers to yellow because the fibers usually contain a hindered phenol antioxidant. The hindered phenol antioxidant increases long term stability but accelerates yellowing. It is known that a hindered phenol antioxidant and a thiodipropionate are most effective when used together. Certain HALS provide not only excellent uv stability but also such good long term thermal stability that the PP fibers will outlast some of the pigments used to color them. Therefore a HALS is combined with a hindered phenol antioxidant and a phosphite.
Pigments are selected with an eye to their effect on the processing of the PP fibers, the stability requirements of the end product, the pigment's interaction with the other additives to be used, the color requirements, and the cost of producing the pigmented PP fibers. The intense thrust towards using inexpensive PP fibers in the automobile industry where the color red is in high demand decreed that, despite its high cost, Red 144 be used, because of its intense tinctorial strength and color stability; and, that Red 144 be combined with a compatible uv stabilizer. It was found that the most damaging factor in the stability of Red 144-pigmented PP fibers was their interaction with the hindered amine uv stabilizers used.
The commercial use of red PP fibers requires that the color stability of the PP fiber be such that it equals the useful life of a fabric or other article containing the fiber, which article is exposed to heat and light. Because the stabilizers used generally affect color, though they are not regarded as colorants, and pigments may affect thermal and uv light stability even if they are not known to have such activity, one cannot estimate what the net effect of the interactions might be. (see "Influence of Pigments on the Light Stability of Polymers: A Critical Review" by Peter P. Klemchuk, Polymer Photochemistry 3 pg 1-27, 1983).
We continued our tests with numerous combinations of stabilizers in Red 144-pigmented fibers, screening the samples to determine whether an unacceptable level of color loss was obtained before the fibers disintegrated. We measured the degree of degradation of the pigmented fibers both by visual observation, and by "scratch testing" (described herebelow) the surfaces of exposed fibers.
Fiber degradation is a phenomenon which is easily visible to the naked eye upon inspection of a degrading pigmented yarn exposed either in a Weather-O-Meter in presence of moisture, or, to bright sun (tests are conducted in the Florida sun) under ambient conditions of humidity. Unstabilized Red 114-pigmented PP fibers exposed to the Florida sun show no fading because the pigmented fibers degrade far more rapidly than the pigment, which results in continual sloughing off of layers of fiber exposing bright undegraded pigment. Degradation of stabilized PP fibers is characterized (i) by a fuzzy, peach-skin-like appearance of the surface of the fabric (made with the pigmented fibers), and (ii) the problem of fading color.
Of particular interest is the peculiar uv-stabilization effect of N-(substituted) .alpha.-(3,5-dialkyl-4-hydroxy-phenyl)-.alpha.,.alpha.-disubstituted acetamides in which one of the substituents on the N atom is a 2-piperazinone group. More correctly, the compounds are "N-(substituted)-1-(piperazin-2-one alkyl)- c -(3,5-dialkyl-4-hydroxyphenyl)-.alpha.,.alpha.-substituted acetamides", which are hereinafter referred to as "3,5-DHPZNA" for brevity. This 3,5-DHPZNA stabilizer is disclosed in U.S. Pat. No. 4,780,495 to John T. Lai, for its uv-light stabilization in PP, and, because of the presence of a polysubstituted piperazinone (PSP) group in the molecule, was routinely tested in PP plaques for such stabilization-effectiveness as 3,5-DHPZNA might have. Since the majority of PP articles are extruded or molded shapes other than fibers, most testing for stabilization is conventionally done with plaques, not fibers, because plaques are more conveniently prepared. The plaques deteriorated rapidly. Only by chance was 3,5-DHPZNA also tested in Red 144-pigmented PP fibers, and its remarkable effectiveness noted.
As one would expect, some pigments enhance heat and light stability of PP fibers stabilized with a particular antioxidant and hindered amine stabilizer. Other pigments have the opposite effect. Until tested, one cannot predict with reasonable certainty, what the effect will be. For example, with a nickel-containing stabilizer, Red 101 (iron oxide) is a prodegradant. With the more effective hindered amine stabilizers, both Yellow 93 and Red 144 are prodegradants. The effect of these pigments in stabilized PP fibers could not have ben predicted by their behavior in unstabilized pigmented fibers, or by their behavior with a different stabilizer. With a nickel-containing stabilizer, Red 144 (unlike Red 101) is a stabilizer (not a prodegradant), but Red 144 is a prodegradant with Tinuvin 770. Yellow 93, a stabilizer when no other stabilizer is present, is neutral with nickel stabilization but is a prodegradant with Tinuvin 770 (see "Stabilization of Polypropylene Fibers" by Marvin Wishman of Phillips Fibers Corporation). Specifically with respect to red PP fibers, the problem was to find a combination of stabilizers which circumvented the proclivity of Red 144 to degrade the PP fibers when the pigment is combined with a conventional AO and uv light stabilizer. Because Red 144 was a prodegradant it seemed desirable to use only as much of it as would provide the desired tinctorial effect for the required period of time, namely the useful life of the stabilized PP fiber.
The effect of a large number of pigments on the stability of PP fibers stabilized with Tinuvin 770 has been reported by Steinlin and Saar (see "Influence of Pigments on the Degradation of Polypropylene Fibers on Exposure to Light and Weather", paper presented at the 19th International Manmade Fiber Conference, September 1980 in Austria).
In the same vein, like other workers before us, we tested a large number of combinations with Red 144, and tested them in fibers. We confirmed that Tinuvin 144 in combination with Red 144, stabilizes fiber but does not stabilize the red color, acting more like a prodegradant for color stability. Tinuvin 144 is a HALS molecule of comparable size to that of 3,5-DHPZNA, and like 3,5-DHPZNA is a hybrid molecule. Tinuvin 144 combines a hindered phenol with a substituted piperidyl rather than with a substituted piperazinone. But this combination of hindered phenol and piperidyl groups in one molecule is not as effective with Red 144 as the combination of hindered phenol and piperazinone. Chimassorb 944 provides excellent stabilization to Red 144-pigmented PP fibers, but Chimassorb 944, like Tinuvin 144, provide excellent uv stabilization only of the PP, not the color, which degrades rapidly. With Tinuvin 770, there is greater negative interaction than with Tinuvin 144 as evidenced by decreased stability of the PP.
Generally, if a stabilizer is effective in fibers it is effective in plaques, but the opposite is not true. Red 144-pigmented PP fibers are stabilized with 3,5-DHPZNA against heat and light and it is reasonable to expect a comparable effect in Red 144-pigmented PP plaques. Moreover, 3,5-DHPZNA-stabilized PP fibers pigmented with Red 144 do not require the added presence of a conventional hindered phenol antioxidant, though a small amount up to about 0.1 phr, may be used to provide a high level of melt-stability when the Red 144-pigmented PP is extruded from a spinneret.
U.S. Pat. No. 4,797,438 to Kletecka et al discloses that hindered amines with a specific structure known to exhibit excellent uv stabilization in numerous host polymers without notably distinguishing one polymer from another as far as their relative susceptibility to uv stabilization is concerned, are surprisingly effective to stabilize PP against degradation by gamma-radiation. Moreover, such stabilization extends to articles of arbitrary shape, including fibers, and these amines are more effective when used without an AO, phosphite or thioester. It was not known, however, nor could we reasonably predict, what the interaction of the 3,5-DHPZNA stabilizer in particular, would be with Red 144 pigment.
The peculiarly distinguishing structural feature of the stabilizers in the '438 Kletecka et al composition, is that they, like 3,5-DHPZNA, contain as an essential portion of their basic structure, a PSP having an N.sup.1 -adjacent carbonyl in the PSP group, and at least the C.sup.3 (carbon atom in the 3-position in the ring) has two substituents (hence "polysubstituted or substituted"), which may be cyclizable, that is, form a cyclic substituent. But unlike 3,5-DHPZNA, those stabilizers do not contain a hindered phenol group in the same molecule.
Though 3,5-DHPZNA compounds referred to in the aforementioned '495 Lai patent were known to be excellent UV stabilizers in colorless organic materials when used in combination with antioxidants, there was nothing to suggest that its incorporation in PP fibers, alone among other polymers tested, in the presence of less than 0.1 phr of each of a conventional hindered phenol antioxidant and phosphite which provide melt-stabilization, would provide effective stabilization against discoloration of Red 144 pigment.